Wheel lock control system

ABSTRACT

A wheel lock control system for cyclically releasing and applying the wheel brakes to prevent wheel lock during braking. A deceleration switch produces a control signal to effect a wheel brake release when the wheel speed exceeds a predetermined deceleration threshold representing an incipient wheel lock condition. A cycle depth integrator provides a cycle depth signal that is representative of the magnitude of the change of wheel speed after a brake release. A cycle depth comparator is responsive to the control signal and the cycle depth signal to effect wheel brake release during the control signal and to maintain the wheel brake release as a function of the cycle depth signal magnitude and duration and wheel speed characteristics to provide for adaptation to all vehicle and road conditions. A reset circuit cooperates with the cycle depth integrator to maintain a release to provide for wheel speed recovery when the average wheel deceleration exceeds a maximum possible vehicle deceleration after a time period dependent upon the magnitude of the average wheel deceleration and the time period that the average wheel deceleration exceeds the maximum possible vehicle deceleration.

This invention relates to a wheel lock control system for vehicle brakeswhich is adaptive to all vehicle load and road conditions.

U.S. Pat. No. 3,953,080, which is assigned to the assignee of thisinvention, sets forth a wheel lock control system particularly adaptedto provide wheel lock control for heavy duty trucks with air brakes andwhich is adaptive to various vehicle and road conditions. The generalobject of this invention is to provide an improved wheel lock controlsystem which is adaptive to all vehicle and road conditions.

It is another object of this invention to provide a wheel lock controlsystem which initiates a brake release as a function of the vehiclewheel deceleration and provides for brake reapplication as a function ofthe magnitude and duration of the wheel speed departure from the wheelspeed at the time of brake release.

It is another object of this invention to provide a wheel lock controlsystem having three control modes, each of which is operable undercertain vehicle and road conditions to provide optimum wheel lockcontrol.

It is another object of this invention to provide a wheel lock controlsystem which operates as a deceleration switch on certain vehicle androad conditions, as a wheel recovery system on other vehicle and roadconditions, and as a wheel acceleration switching circuit on othervehicle and road conditions.

It is another object of this invention to provide a wheel lock controlsystem for providing wheel spaced recovery when the average wheeldeceleration exceeds a maximum possible vehicle deceleration.

The wheel lock control system of this invention is comprised of acircuit which has three modes of wheel lock control operation to provideoptimum control of the wheel brakes during braking for all vehicle loadand road surface conditions. A deceleration switch functions in allthree modes to initiate a brake release when the wheel decelerationexceeds a reference deceleration level.

In the first mode of control, the duration of the brake release periodis controlled by the deceleration switch which effects reapplication ofthe wheel brakes when the wheel speed deviation from the integral of thereference deceleration during release returns to zero. This mode ofoperation is generally associated with heavy vehicle load and/or highcoefficient of friction road surface conditions.

In the second mode of control, the duration of the brake release iscontrolled by a comparison of the output of a cycle depth integratorwith a cycle depth reference having a magnitude determined by wheelspeed, wheel acceleration, and average wheel deceleration. The output ofthe cycle depth integrator is a cycle depth signal which is a compositesignal comprised of a first portion representing the amount that thetime integral of a deceleration reference signal exceeds the wheel speedduring brake application, or stated another way, the amount that thewheel speed decreases below a reference speed decelerating at thereference deceleration, and a second portion which is the integral ofwheel acceleration during brake release. The second decelerationreference is approximately equal to the reference deceleration levelassociated with the deceleration switch and generally represents amaximum possible vehicle deceleration level. During normal wheel lockcontrol operation, the average wheel deceleration is equal to or lessthan the reference deceleration so that the cycle depth signal has amagnitude that generally represents the wheel speed change during abrake release period (hereinafter referred to as the cycle depth).

The duration of the brake release in the second mode of operationprovides for wheel speed recovery and is a function of the magnitude andduration of the cycle depth and the magnitude of the cycle depthreference. This mode of operation is generally associated with mediumvehicle load and/or medium coefficient of friction road surfaceconditions.

In the third mode of control, the duration of the brake release iscontrolled by a release inhibit circuit which is responsive to wheelacceleration to effect brake reapplication when the wheel accelerationdecreases to a small value representing the wheel speed approachingvehicle speed. This mode of operation is generally associated with lightvehicle loads and/or low coefficient of friction road surfaces.

A reset circuit is provided to reset the cycle depth signal at theoutput of the cycle depth integrator upon each brake reapplication to avalue equal to the value the cycle depth signal would have obtained ifthe second deceleration reference were continually summed with the wheelacceleration signal throughout both the release and apply periods. Whenthe average wheel deceleration is less than the reference deceleration,the cycle depth signal is reset to zero. However, when the average wheeldeceleration exceeds the maximum possible vehicle deceleration,indicating that the wheel is approaching a wheel lock condition, thecycle depth signal has a magnitude that increases with each resetprovided by the reset circuit until it attains a value during a brakerelease period that is greater than the cycle depth reference tomaintain the wheel brake released and allow the wheel speed to increasetoward vehicle speed to thereby prevent a wheel lock condition.

The invention may be best understood by reference to the followingdescription of a preferred embodiment and the drawings in which:

FIG. 1 is a block diagram of the wheel lock control system incorporatingthe principles of this invention;

FIGS. 2a and 2b is a schematic electrical diagram of the wheel lockcontrol system of FIG. 1; and FIGS. 3a, 3b, 3c and 3d are graphsrepresenting the operation of the three operating modes of the wheellock control system of FIGS. 1 and 2.

The following description of a preferred embodiment is directed towardthe invention as applied to heavy duty trucks with air brakes. However,the invention is considered to be of general application for all wheeledvehicles and braking systems.

In the preferred embodiment of this invention it is contemplated thateach axle of the vehicle is controlled independently of the other axles,both on the tractor and on the trailer, such that each axle will befurnished with a complete wheel lock control system, the brake systemson the several axles having in common only the manually controlled airpressure which is supplied at the will of the vehicle operator. However,it is understood that the invention contemplates the control of all thebrakes on tandem axles with a single wheel lock control system or anyother desired wheel-axle-control system combinations.

The system incorporating the principles of this invention follows thewell established principle of sensing incipient wheel lock when brakepressure is applied to the vehicle brakes by the vehicle operator,releasing the brake pressure for a duration determined by the system,and reapplying the pressure. This cycle is repeated as necessary toachieve the desired braking action.

The term "acceleration" as used herein refers to both positiveacceleration and deceleration unless otherwise specified. Further, theterm "cycle depth" refers to the magnitude of wheel speed deviationduring a wheel brake release period from the wheel speed at the time ofwheel brake release.

Referring to FIG. 1, vehicle wheels 10 and 12 on opposed sides of avehicle axle are connected with speed sensors 14 and 16, respectively,for sensing the wheel speeds. The speed sensors 14 and 16 are preferablytoothed wheel variable reluctance electromagnetic transducers providingrespective alternating signals having frequencies proportional to thewheel speeds. These signals are connected to squaring amplifiers 18 and20, respectively, which provide square wave signals having a frequencyequal to the frequency of the alternating signals from the speed sensors14 and 16. The square wave signals from the squaring amplifiers 18 and20 are coupled to respective tachometer circuits 22 and 24 which provideelectrical analog signals having a magnitude proportional to the speedof the wheels 10 and 12, respectively. These analog signals are suppliedto a speed selector 26 which transmits the analog signal representingthe lowest wheel speed to a conductor 28.

The wheel speed signal on line 28 is supplied to a deceleration switch30, a cycle depth integrator 32, a deceleration amplifier 34, anacceleration switch 36, a release inhibit switch 38 and the negativeinput of a cycle depth comparator 40.

The cycle depth comparator 40 generates a brake release signal when themagnitude of the signals to its positive input exceeds the magnitude ofthe signals to its negative input. The brake release signal is suppliedto a solenoid driver and shutdown circuit 42 which energizes a releasesolenoid 44 in response thereto to effect a wheel brake release for theduration of the brake release signal.

The deceleration switch 30 generates a control signal when the wheeldeceleration exceeds a reference deceleration representing an incipientwheel lock condition. The reference deceleration is approximately equalto the maximum possible vehicle deceleration and is made variable as afunction of wheel speed. In this embodiment, it will be assumed that thereference deceleration representing incipient wheel lock is 0.9g at zerowheel speed and 1.3g at 60 mph wheel speed. The deceleration switch 30terminates the control signal when the wheel speed deviation from theintegral of the reference deceleration during release approaches zero.The control signal is coupled to the positive input of the cycle depthcomparator 40.

The cycle depth integrator 32 provides a cycle depth signal to thepositive input of the cycle depth comparator 40 which is a compositesignal comprised of a first portion representing the amount that thetime integral of a reference deceleration exceeds the wheel speed duringbrake application and a second portion which is the integral of wheelacceleration during brake release. The deceleration reference for thecycle depth integrator 32 is approximately equal to the decelerationreference in the deceleration switch 30 so that the cycle depth signalfrom the cycle depth integrator 32 generally represents the magnitude ofwheel speed change (the cycle depth) during a brake release period. Inthis embodiment, it will be assumed that this deceleration reference is1.0g. The brake release signal from the cycle depth comparator 40 iseffective to terminate the deceleration reference in the cycle depthintegrator 32 for its duration. When the release signal from the cycledepth comparator 40 is terminated to effect wheel brake reapplication,the cycle depth signal is set to the value equal to the amount that thetime integral of the reference deceleration exceeds wheel speed duringboth brake application and release periods. When the average wheeldeceleration is less than the reference deceleration, the cycle depthsignal is set to zero. When the average wheel deceleration is greaterthan the reference deceleration, the cycle depth signal is set to acorresponding value as previously described.

The deceleration amplifier 34 provides a cycle depth reference signalwhose magnitude is proportional to deceleration until it saturates at apredetermined deceleration level less than the deceleration reference inthe deceleration switch 30. For example, in this embodiment, thedeceleration amplifier 34 saturates when the wheel deceleration reachesapproximately 0.8g. The scaling of the deceleration amplifier 34 is suchthat the control signal from the deceleration switch 30 will alwaysrevert to its low state before the deceleration amplifier 34 outputshifts to a low state. This will insure that the cycle depth referencesignal output of the deceleration amplifier 34 will extend beyond thetermination of the control signal from the deceleration switch 30. Thecycle depth reference output of the deceleration amplifier 34 is coupledto one input of a gate 46.

The acceleration switch 36 provides a cycle depth reference outputsignal upon the detection of a predetermined high level of positivewheel acceleration. For example, in this embodiment, the accelerationswitch 36 provides the cycle depth reference signal when the wheelpositive acceleration reaches approximately 2.0g's. The output of theacceleration switch 36 is coupled to a second input of the gate 46 whoseoutput is coupled to the negative input of the cycle depth comparator40.

The gate 46 functions to couple the output cycle depth reference signalfrom the deceleration amplifier 34 or the acceleration switch 36 havingthe greatest magnitude to the negative input of the cycle depthcomparator 40. The sum of this signal and the wheel speed signal coupledto the negative input of the cycle depth comparator 40 represents acycle depth reference to be compared with the cycle depth represented bythe cycle depth signal from the cycle depth integrator 32. In thisembodiment, the saturated output of the deceleration amplifier 34represents a cycle depth of 3 mph and the output of the accelerationswitch 36 upon the detection of the positive acceleration levelrepresents a cycle depth of 3 mph. Further, the wheel speed input to thenegative terminal of the cycle depth comparator 40 from line 28 providesa component of the cycle depth reference which is approximately equal to1.5 mph for every 10 mph of wheel speed.

The release inhibit switch 38 is a "normally on" switch providing arelease inhibit signal to the negative terminal of the cycle depthcomparator 40. The release inhibit signal is terminated by a controlsignal generated by the deceleration switch 30 or upon the detection ofa positive acceleration greater than a predetermined low accelerationlevel which is made variable as a function of the cycle depth signalmagnitude. For example, in this embodiment, the release inhibit switch38 terminates the release inhibit signal upon the detection of 0.5gpositive acceleration at zero magnitude of the cycle depth signal and0.1g for maximum cycle depth signal magnitudes. During the absence of acontrol signal and when the positive wheel acceleration decreases tobelow the predetermined low level, the release inhibit signal from therelease inhibit switch is again generated.

The magnitude of the release inhibit signal from the release inhibitswitch 38 is greater than the maximum magnitude of the cycle depthsignal from the cycle depth integrator 32. Therefore, a brake releasesignal can be initiated at the output of the cycle depth comparator 40only by the control signal generated by the deceleration switch 30 whenthe wheel deceleration exceeds the deceleration reference of thedeceleration switch 30 representing an incipient wheel lock condition.Further, the magnitude of the control signal from the decelerationswitch 30 is greater than the maximum cycle depth reference signal inputto the cycle depth comparator 40 so that the control signal is alwayseffective to provide a brake release signal and therefore a brakerelease for its duration.

After the release has been initiated by the control signal, the systemresponse changes as a function of the length and depth of the wheelcycle as represented by the cycle depth signal from the cycle depthintegrator 32, the deceleration of the vehicle, and the acceleration ofthe vehicle wheels.

For heavy vehicle loading and/or high coefficient surfaces, where thecycle depth is typically short and shallow, the controller will run onlyon the action of the deceleration switch 30. In this mode of operation,the termination of the control signal generated by the decelerationswitch 30 is effective to provide for brake reapplication. This mode ofoperation provides optimum performance for high vehicle loading and/orhigh coefficient of friction road surfaces.

On medium coefficient of friction surfaces and/or for medium vehicleloading where the wheel speed cycle depth is larger, the cycle depthsignal from the cycle depth integrator 32 in conjunction with the cycledepth references supplied to the negative input of the cycle depthcomparator 40 control the duration of the brake release signal. In thismode of operation, the cycle depth signal has a magnitude greater thanthe cycle depth reference signals supplied to the negative input of thecycle depth comparator 40 upon termination of the control signal fromthe deceleration switch 30 and the brake release signal is extendeduntil the cycle depth decreases to below the reference cycle depth. Thismode of operation provides for optimum performance on medium coefficientof friction road surfaces and/or for medium vehicle load conditions.

For light vehicle loading and/or low coefficient of friction roadsurfaces, the release inhibit switch 38 provides a third mode ofoperation wherein the cycle depth signal from the cycle depth integrator32 maintains a release until the inhibit switch 38 forces a brakereapplication when the positive wheel acceleration decreases to a lowlevel representing that the wheel is approaching vehicle speed. At thislow positive wheel acceleration level, the release inhibit switch 38generates the release inhibit signal to effect termination of the brakerelease signal and initiate wheel brake reapplication. This mode ofoperation provides optimum performance for low vehicle loads and/or lowcoefficient of friction road surfaces.

The remainder of the circuit of FIG. 1 provides a self-check service tomonitor malfunctions in the wheel lock control system and to provide forsystem shutdown and warning upon the occurrence of the malfunction.

The self-check circuit includes a sensor check circuit 48 which isresponsive to the speed sensors 14 and 16 to provide a fault signal to atimer 50 when an open circuit in either of the sensors 14 and 16 isdetected. Another fault signal is provided to the timer 50 from thesystem power supply 52 when the vehicle battery voltage B+ decreases toa level below the wheel lock control system regulated voltage Z+. Thesolenoid driver and shutdown circuit 42 also provides a signal to thetimer 50 when an open or short circuit to ground occurs in the releasesolenoid 44 or during the period that the release solenoid 44 isenergized to release the wheel brakes.

The timer provides an output at the trigger level of a comparatorcircuit 54 when one of the fault signals supplied thereto from thesensor check 48 or the power supply 52 has a duration that exceeds apredetermined time period or when the release solenoid is shorted toground or open circuited or is energized to release the wheel brakes fora period extending beyond a predetermined time limit which is variableas a function of the cycle depth signal from the cycle depth integrator32.

The comparator 54 is responsive to the output of the timer 50 attainingthe trigger level to generate a shutdown signal which is supplied to thesolenoid driver and shutdown circuit 42 to deenergize the releasesolenoid 44 and terminate a brake release or to prevent the energizationof the release solenoid and thereby prevent initiation of a brakerelease.

To provide for fault conditions which may be self-correcting while yetpreventing the wheel lock control logic from releasing the vehiclebrakes at any time during a brake release signal once a fault isdetected during the brake release signal, the comparator 54 is madelatching or unlatching as a function of whether or not the wheel lockcontrol system is commanding a release.

The comparator 54 receives a signal from the solenoid driver andshutdown circuit 42 representing the wheel lock control system output.If a fault occurs and thereafter self corrects while the wheel lockcontrol system is in a brake apply mode, the shutdown signal from thecomparator 54 exists only for the duration of the fault. However, if thefault coexists even momentarily with a brake release signal from thecycle depth comparator 40, the shutdown signal is generated by thecomparator 54 for the duration of the longer one of the fault conditionor the brake release signal. In this manner, the brake release signal iseffective to latch the comparator 54 to provide the shutdown signal forits duration. Upon termination of the shutdown signal, the wheel lockcontrol circuit is again operative to provide wheel lock control.

The fault signal from the comparator is supplied to a lamp warningcircuit 56 to provide an indication of the fault condition.

To provide for a check of the sensors 14 and 16 that occurs for reasonsother than loss of continuity, a dynamic sensor check 58 is providedwhich monitors the wheel speeds from the tachometers 22 and 24 andgenerates an output supplied to the cycle depth comparator 50 toinitiate a brake release when one wheel speed exceeds the other by apredetermined amount, which, in this embodiment, is 17 mph. The outputof the sensor check 58 will maintain the release beyond the maximum timelimit and until the self-check circuit previously described provides ashutdown signal for disabling the wheel lock control circuit.

Referring to FIGS. 2a and 2b, a specific example of the system of FIG. 1is set forth.

The regulated power supply 56 provides a regulated voltage Z+ foroperating the wheel lock control system. The unregulated voltage B+ ofthe vehicle battery is coupled across a series circuit including aresistor 60, a Zener diode 62 and a resistor 64. The Zener diodeprovides a regulated voltage which is coupled to the base of an NPNtransistor 66 whose collector is coupled to the battery supply B+. Thetransistor 66 is biased into conduction to supply the regulated voltageZ+ which is equal to the voltage at the cathode of the Zener diode 62less the base-emitter drop of the transistor 66. The voltage Z+ isdeveloped across a resistor 68.

The remainder portion of the regulated power supply 52 relates to theprovision of a fault signal when battery voltage B+ becomes less thanthe desired regulated voltage Z+, which condition may affect operationof the wheel lock control circuit. This fault signal is provided bymeans of an NPN transistor 70 having its emitter grounded and having itscollector electrode coupled to the emitter of the transistor 66 througha resistor 72. When the battery voltage B+ exceeds the breakdown voltageof the Zener diode 62, the resulting voltage drop across the resistor 64is sufficient to bias the transistor 70 conductive so that the potentialat its collector is at or near ground potential. However, if the batteryvoltage B+ should decrease to below the breakdown voltage of the Zenerdiode 62, the transistor 70 is biased nonconductive and the voltage atits collector is increased to approximately the battery voltage B+ tosupply a signal through a diode 73 representing the fault condition.This fault signal is supplied to the self-check timer 50 through aconductor 74.

The squaring amplifier 18 includes a resistor 76 which is series coupledwith the output coil of the speed sensor 14 between the regulatedvoltage Z+ and ground. The alternating output from the speed sensor 14representing the speed of the vehicle wheel 10 on one side of thevehicle axle is coupled to the positive input of an operationalamplifier 78 through series coupled resistors 80 and 82. The regulatedvoltage Z+ is coupled to the junction between the resistors 80 and 82through a capacitor 84 and to the positive input of the amplifier 78through a resistor 86. A bias is provided to the negative input of theamplifier 78 by means of a voltage divider comprised of resistors 88 and90 series coupled between the regulated voltage Z+ and ground. Afeedback resistor 92 is coupled between the output of the amplifier 78and its positive input terminal.

The amplifier 78 and the remaining operational amplifiers in the circuitof FIGS. 2a and 2b are current amplifiers wherein the current input tothe positive terminal is subtracted from the circuit input to thenegative terminal and wherein a positive output is provided when thecurrent input to the positive terminal exceeds the current input to thenegative terminal. Further, the operational amplifiers have a normallyhigh output when no current is supplied to the input terminals. The biascurrent supplied by the voltage divider comprised of the resistors 88and 90 to the negative input of the amplifier 78 functions to bias theoutput of the amplifier 78 at ground potential. The squaring amplifier18 is responsive to the alternating signal input from the speed sensor14 to provide a square wave output having the same frequency.

The squaring amplifier 20 is comprised of the resistors 94, 96, 98, 100,102, 104 and 106, a capacitor 108 and an operational amplifier 110 whichis connected in identical manner as the squaring amplifier 18 to providea square wave output having a frequency equal to the frequency of thealternating signal supplied by the speed sensor 16 and corresponding tothe speed of the vehicle wheel 12.

The square wave output of the squaring amplifier 18 is supplied to adifferentiating circuit in the tachometer 22 comprised of a resistor 112and a capacitor 114. A positive current pulse is supplied to thepositive input of an operational amplifier 116 through a diode 118 oneach leading edge of each square wave signal from the squaring amplifier18, and a negative current pulse is supplied to the negative input ofthe operational amplifier 116 through a diode 120 on the trailing edgeof each square wave signal from the squaring amplifier 18. The negativeinput of the operational amplifier 116 is grounded through a capacitor122. A feedback capacitor 124 is coupled between the output of theoperational amplifier 116 and the negative input terminal thereof toprovide for an integrator having an integrating scaling determined by afeedback resistor 126 parallel coupled with the capacitor 124. Theoutput of the amplifier 116 is an analog voltage having a magnitudedirectly proportional to the speed of the vehicle wheel 10.

The tachometer 24 includes the resistors 128 and 130, the capacitors132, 134 and 136, diodes 138 and 140 and the operational amplifier 142coupled in identical manner as the tachometer circuit 22. The output ofthe tachometer 24 is an analog voltage having a magnitude directlyproportional to the speed of the vehicle wheel 12.

The speed selector 26 is responsive to the wheel speed signals from thetachometer circuit 22 and the tachometer circuit 24 and provides a wheelspeed signal output on conductor 28 which is equal to the speed of theslowest vehicle wheel 10 or 12. The speed selector 26 includes the PNPtransistors 144 and 146 having their collectors coupled to ground andtheir emitters coupled to the regulated voltage Z+ through a resistor148. The wheel speed signal from the tachometer 22 is coupled to thebase of the transistor 144 and the wheel speed signal from thetachometer 24 is coupled to the base of the transistor 146. The outputof the speed selector 26 on line 28 is the wheel speed signal having amagnitude equal to the output of the tachometer 22 or 24 associated withthe slowest vehicle wheel plus the base-emitter voltage drop of thetransistor 144 or 146 that is conducting.

The deceleration switch 30 includes a filter resistor 150 series coupledwith a differentiating capacitor 152 between the conductor 28 carryingthe wheel speed signal and a summing junction 154. The input to thesumming junction 154 from the capacitor 152 is a current having amagnitude representing wheel acceleration. This acceleration signal issummed at the summing junction 154 with a current having a magnituderepresenting a reference deceleration that is representative of amaximum possible vehicle deceleration. This reference decelerationsignal is comprised of two portions. The first portion is a constantcurrent supplied to the summing junction 154 from the regulated voltageZ+ through a resistor 156 and the second portion is a current having amagnitude proportional to wheel speed which is supplied to the summingjunction 154 through a resistor 158 coupled to the line 28 carrying thewheel speed signal. The reference deceleration signal supplied to thesumming junction 154 represents a wheel deceleration of 0.9g at zerowheel speed increasing to 1.3g at 60 mph wheel speed.

The output of the summing junction 154 representing the sum of the wheelacceleration and the reference deceleration signal is supplied to thenegative input of an operational amplifier 160 through a resistor 162and a diode 164. A diode 166 is coupled between the anode of the diode164 and the output of the operational amplifier 160 to remove a speedthreshold introduced by the diode 164. The positive input terminal ofthe operational amplifier 160 is grounded. A feedback filteringcapacitor 168 is coupled between the output of the operational amplifier160 and the summing junction 154.

In the absence of wheel deceleration and for wheel decelerations lessthan the reference deceleration, the reference deceleration currentsupplied to the summing junction 154 through the resistors 156 and 158and coupled to the negative input of the operational amplifier 160 issufficient to bias the operational amplifier output at ground potential.During periods of wheel deceleration, current through the capacitor 152removes current from the summing junction 154 with the amount of currentthrough the capacitor being a function of the magnitude of wheeldeceleration. When the wheel deceleration is such that the currentthrough the capacitor 152 equals the reference deceleration current(wheel deceleration equaling the reference deceleration representing anincipient wheel lock condition), the output of the operational amplifier160 shifts to a positive voltage level which comprises the controlsignal output of the deceleration switch 30. The resistor 162 functionsto set a value of wheel speed change required before the input to thenegative terminal of the operational amplifier 160 can be reduced tozero to initiate the control signal.

The control signal is coupled to the release inhibit circuit 38 througha diode 170 to terminate the release inhibit signal and is supplied tothe cycle depth comparator 40 through a diode 172 to effect wheel brakerelease.

Due to the delays in the brake system, the brake pressure will continueto increase after the control signal is provided by the decelerationswitch 30 causing a wheel deceleration greater than the referencedeceleration threshold. The decreasing speed signal on the conductor 28decreases the voltage at the summing junction 154 while the decelerationreference current supplied to the summing junction tends to restore thevoltage at the summing junction 154 at approximately the decelerationthreshold rate. The decreased voltage at the summing junction 154represents the magnitude of departure of the wheel speed on line 28 froma reference speed decelerating at approximately the decelerationthreshold rate. As the wheel deceleration stops and the wheel begins toaccelerate, the junction 154 voltage rises in response to the wheelspeed departure from the time integral of the reference decelerationapproaching zero, at which point current is supplied to the negativeinput of the operational amplifier 160 to terminate the control signal.

The control signal is supplied to the positive input of an operationalamplifier 174 in the cycle depth comparator 40 through a scalingresistor 176.

The cycle depth integrator 32 includes a filter resistor 178 seriescoupled with a differentiating capacitor 180 between the conductor 28carrying the wheel speed signal and a summing junction 182. The input tothe summing junction 182 from the capacitor 180 is a current having amagnitude representing wheel acceleration. This acceleration signal isselectively summed at the junction 182 with a current representing thereference deceleration of 1.0g which is approximately equal to thereference deceleration in the deceleration switch 30 and whichrepresents the maximum possible vehicle deceleration. The referencedeceleration current is selectively provided by a PNP transistor 184,and resistors 186 and 188 series coupled between the junction 182 andthe regulated voltage Z+. A capacitor 190 is coupled in parallel withthe resistor 188.

The reference deceleration current is supplied to the summing junction182 by the transistor 184 when it is biased conductive.

The sum of the vehicle acceleration signal and the decelerationreference signal is supplied to the negative input of an operationalamplifier 192 through a diode 194. The positive input terminal of theoperational amplifier 192 is grounded. A diode 196 is coupled betweenthe anode of the diode 194 and the output of the operational amplifier192 to remove the speed threshold introduced by the diode 194. Theoperational amplifier 192 has a feedback capacitor 198 coupled betweenits output and the summing junction 182 to form an integrator.

The output of the operational amplifier 192 is normally held at groundpotential by the deceleration reference current supplied to the junction182. However, when the current away from the junction 182 through thecapacitor 180 resulting from wheel deceleration exceeds the current intothe junction 182 through the transistor 184, the output of the amplifieris the cycle depth signal which is the integral of the summation of thecurrents until the integral again attains zero or ground potential whichdefines a limit of integration.

The conduction of the transistor 184 is controlled by the brake releasesignal from the output of the cycle depth comparator 40 which is coupledto the base electrode of the transistor 184 through a resistor 200.During the absence of a brake release signal, the output of the cycledepth comparator is ground potential and the transistor 184 is biasedinto conduction to supply the deceleration reference current to thesumming junction 182. Upon the generation of a brake release signal, thetransistor 184 is biased off so that the cycle depth integratorintegrates only the wheel acceleration signal supplied to the summingjunction 182 through the capacitor 180. The cycle depth signal istherefore a composite signal comprised of a first portion representingthe amount that the time integral of the deceleration reference signalexceeds wheel speed during brake application and a second portion whichis the integral of wheel acceleration during brake release, thecomposite signal having a lower integration limit of zero.

Since the deceleration reference signal supplied to the summing junction182 through the transistor 184 is approximately equal to thedeceleration reference supplied to the summing junction 154 in thedeceleration switch 30, the cycle depth signal output of the cycle depthintegrator has a magnitude which generally represents the magnitude ofthe change in wheel speed during the period of the brake release signalsupplied by the cycle depth comparator 40. The cycle depth signalrepresenting the cycle depth is supplied to the positive input of theamplifier 174 in the cycle depth comparator 40 through a scalingresistor 202.

During the period of a brake release, the transistor 184 isnonconducting and the capacitor 190 discharges through the resistor 188.The rate of discharge of the capacitor 190 is releated to thedeceleration reference current supplied to the junction 182 while thetransistor 184 is conducting so that upon the termination of the brakerelease signal, and the resulting conduction of the transistor 184, acurrent pulse is supplied through the resistor 186 resulting from therecharging of the capacitor 190 having an amplitude and duration whichsets the magnitude of the cycle depth signal from the cycle depthintegrator to a value which the cycle depth signal would have obtainedif the reference deceleration were continually supplied to the junction182 through the transistor 184. When the average wheel deceleration isless than the reference deceleration supplied through the resistors 186and 188, the cycle depth signal from the cycle depth integrator is resetto ground potential. However, when the average wheel decelerationexceeds the reference deceleration supplied through the resistors 186and 188, the cycle depth signal is set to the value previouslydescribed. Continued deceleration of the wheel beyond the referencedeceleration is effective to continually increase the level to which thecycle depth signal is reset. This is reflected in the increased currentsupplied to the positive input of the amplifier 174 through the resistor202.

The deceleration amplifier 34 includes an operational amplifier 204having its positive input terminal grounded. A filter resistor 206 and adifferentiating capacitor 208 are series coupled between the conductor28 carrying the wheel speed signal and a junction 210. The currentthrough the capacitor 208 represents wheel acceleration. This signal iscoupled to the negative input of the amplifier 204 through a diode 212.A feedback filter capacitor 214 is coupled between the output of theamplifier 204 to the junction 210. A resistor 216 is parallel coupledwith the capacitor 214.

The deceleration amplifier 34 provides a first portion of a cycle depthreference having a magnitude proportional to vehicle deceleration untilit saturates at a specified deceleration level, which in thisembodiment, is 0.8g. Thereafter, a further wheel deceleration decreasesthe voltage at the junction 210 while the current through the resistor216 flowing into the junction 210 tends to restore the voltage atapproximately the 0.8g level. The voltage at the junction 210 thenrepresents the departure of the vehicle wheel speed from a referencespeed decelerating at approximately the deceleration represented by thefeedback through the resistor 216. As the wheel begins to accelerate andthe voltage at the junction 210 rises in response to the wheel speeddeparture approaching zero, the output of the deceleration amplifier 204again decreases to ground potential. The first portion of the cycledepth reference signal from the deceleration amplifier 34 when atsaturation may represent, for example, a cycle depth of 3 mph. 3

The scaling of the deceleration amplifier 34 is such that the controlsignal generated by the deceleration switch 30 is always terminatedbefore the output of the deceleration amplifier 34 goes out ofsaturation to insure that the first portion of the cycle depth referenceoutput of the deceleration amplifier 34 is at its maximum level of 3 mpheach time that the deceleration switch 30 terminates the control signal.

The acceleration switch 36 includes a filter resistor 218 series coupledwith a differentiating capacitor 220 between a summing junction 221 anda filter 224, which supplies a signal representing wheel speed. Thefilter 224 includes a resistor 226 and a capacitor 228 series coupledbetween the conductor 28 carrying the wheel speed signal and ground. Thevoltage across the capacitor 228 is a filtered wheel speed signal. Thecurrent input to the summing junction 221 through the capacitor 220represents wheel acceleration. A resistor 222 is coupled between thesumming junction 221 and ground. The summing junction 221 is coupled tothe base of a PNP transistor switch 230 through a resistor 232. Theemitter of the transistor switch 230 is coupled to the regulated voltageZ+ and the collector thereof is coupled to the negative input of anoperational amplifier 234 through a resistor 236. Current is supplied tothe positive input of the amplifier 234 from the regulated voltage Z+through a resistor 238.

The base of the transistor switch 230 is coupled to ground through theresistors 222 and 232 so that it is normally conductive. The current outof the summing junction 221 through the resistor 222 has a magnituderepresenting a reference positive wheel acceleration which is 2.0g inthis embodiment. This acceleration reference signal is summed at thesumming junction 221 with the wheel acceleration signal from thedifferentiating capacitor 220. In the absence of wheel acceleration andfor wheel accelerations less than the reference accelerationsrepresented by the current through the resistor 222, the current fromthe summing junction 221 through the resistor 222 is sufficient tomaintain the transistor 230 biased conductive. During periods of wheelacceleration, current through the capacitor 220 supplies current to thesumming junction 221 with the amount of current through the capacitor220 being a function of the magnitude of wheel acceleration. When thewheel acceleration is such that the current through the capacitor 220equals the reference acceleration current (wheel acceleration equalingthe reference acceleration), the transistor switch 230 is biasednonconductive. The resistor 232 functions to set a value of wheel speedchange required before the transistor switch 230 can be biasednonconductive.

The amplifier 234 functions as a current and voltage amplifier and alsoprovides a phase inversion so that its output is a high level forpositive accelerations in excess of the 2.0g reference acceleration.This high level output of the acceleration switch 36 comprises a secondportion of a cycle depth reference signal. The second portion of thecycle depth reference signal from the acceleration switch may representa cycle depth of 3 mph.

The first and second portions of the cycle depth reference outputs ofthe deceleration amplifier 34 and the acceleration switch 36 are coupledto the anode of respective diodes 240 and 242 in the gate 46. Thecathodes of the diodes 240 and 242 are each coupled to the negativeinput of the amplifier 174 in the cycle depth comparator 40 through ascaling resistor 244. The diodes 240 and 242 function to couple thefirst or second portion of cycle depth reference signals from thedeceleration amplifier 34 and the acceleration switch 36 having thegreatest magnitude to the amplifier 174.

The wheel speed signal on line 28 is coupled to the negative input ofthe amplifier 174 of the cycle depth comparator through a scalingresistor 246. The current input to the amplifier 174 through theresistor 246 comprises a third portion of the cycle depth referencesignal and may represent a cycle depth of 1.5 mph for each 10 mph ofwheel speed. The cycle depth reference comprised of the first or secondand the third portions are compared with the cycle depth represented bythe magnitude of the cycle depth signal output of the cycle depthintegrator 32, as will be described, to control the duration of a brakerelease cycle in a particular mode of operation of the wheel lockcontrol system.

The release inhibit circuit 38 includes a filter resistor 245 and adifferentiating capacitor 247 series coupled between the output of thefilter 224 supplying the signal representing wheel speed and thenegative input of an operational amplifier 248. The capacitor 247supplies an acceleration signal to the negative input of the amplifier248. A feedback filtering capacitor 250 is coupled between the output ofthe amplifier 248 and its negative input.

A first portion of a positive acceleration reference signal for therelease inhibit circuit 38 is a constant current supplied from theregulated voltage Z+ to the positive input of the amplifier 248 througha resistor 252 and a second portion of the positive accelerationreference signal is a current having a magnitude proportional to thecycle depth which is supplied to the negative input of the amplifier 248through a scaling resistor 254 coupled to the output of the cycle depthintegrator 32 supplying the cycle depth signal. The net positiveacceleration reference supplied to the inputs of the amplifier 248represents the wheel speed approaching vehicle speed and may, forexample, represent a wheel acceleration of 0.5g when the cycle depth iszero and decreasing to 0.1g when the cycle depth is maximum.

When the wheel is decelerating or has a positive acceleration less thanthe reference acceleration, the amplifier 248 supplies the releaseinhibit signal to the negative input of the amplifier 174 in the cycledepth comparator 40 through a diode 256 and a scaling resistor 258.

The release inhibit signal is terminated when the wheel positiveacceleration exceeds the reference positive acceleration. The releaseinhibit signal is also terminated by the control signal generated by thedeceleration switch 30 which is coupled to the negative input of theamplifier 248 through a scaling resistor 260. The release inhibit signalis again provided upon both the termination of the control signal andwhen the positive wheel acceleration decreases to the low positiveacceleration representing the wheel speed approaching vehicle speed.

In addition to the elements previously described, the cycle depthcomparator 40 includes a resistor 262 coupled between the regulatedvoltage Z+ and the positive terminal of the amplifier 174 to remove thewheel speed bias introduced by the emitter-base voltage drop of thetransistors 144 and 146 in the speed selector 26. A feedback resistor264 is coupled between the positive input and the output of theamplifier 174 which provides a feedback current during a brake releasesignal that is equal to the third portion of the cycle depth reference,which is proportional to wheel speed, when the wheel speed is at, forexample, 25 mph.

The scaling of the resistors 176, 202, 244, 246 and 258 is such that thecurrent supplied to the negative input of the amplifier 174 through theresistor 258 during a release inhibit signal provided by the releaseinhibit circuit 38 is greater than the magnitude of the current suppliedto the positive input through the resistor 202 during maximum values ofthe cycle depth signal provided by the cycle depth integrator 32.Further, the current to the positive input through the resistor 176during a control signal provided by the deceleration switch is greaterthan the maximum total of the cycle depth reference currents supplied tothe negative input through the resistors 244 and 246. Therefore, thecycle depth integrator 32 is ineffective to initiate a brake release andthe deceleration switch 30 is singularly effective to initiate thegeneration of the brake release signal and the resulting brake releaseupon detection of an incipient wheel lock condition and is effective tomaintain the brakes released for the duration of the control signal.Also, the release inhibit signal generated by the release inhibitcircuit 38 is always effective to command brake application.

The output driver and shutdown circuit 34 includes a Darlingtontransistor 266 which is controlled by the brake release signal and aDarlington transistor 268 which is controlled to provide for shutdown ofthe wheel lock control system and to prevent a brake release upon thedetection of certain system faults. The transistors 266 and 268 areseries coupled across the vehicle battery B+ through a resistor 270, aresistor 272 and a resistor 274. The emitter of the transistor 266 iscoupled to the base of an NPN transistor 276 whose emitter is groundedand whose collector is coupled to one side of the release solenoid 44.The junction between the resistors 270 and 272 is coupled to the base ofa PNP transistor 278 whose emitter is coupled to the battery voltage B+and whose collector is coupled to the remaining side of the releasesolenoid 44. A resistor 280 is coupled across the emitter and collectorterminals of the transistor 278.

The shutdown transistor 268 normally receives a positive voltage througha conductor 282 and a resistor 284 from the self-check circuit duringthe absence of system faults. The tansistor 268 is therefore normallybiased conductive during normal operation of the wheel lock controlcircuit.

The brake release signal from the cycle depth comparator 40 is coupledto the base of the transistor 266 through a resistor 268. The brakerelease signal is effective to bias the transistor 266 conductive whichin turn biases the transistor 276 conductive to apply ground potentialto one side of the solenoid 44. Simultaneously, current through theresistors 270 and 272 is effective to provide a voltage which biases thetransistor 278 conductive to apply the battery voltage B+ to the otherside of the release solenoid 44 which is thereby energized to effectrelease of the vehicle brakes for the duration of the release signalfrom the cycle depth comparator 40. Upon termination of the brakerelease signal, the transistor 266 is biased nonconductive to deenergizethe release solenoid 44 and thereby effect brake reapplication.

The operation of the wheel lock control logic portion of the circuit ofFIGS. 2a and 2b will be described with reference to the FIGS. 3a - 3dwhich illustrate the wheel lock control operation in its three operationmodes. In each of the FIGS. 3a - 3d, curve A represents the magnitude ofthe wheel speed change after the wheel brakes have been released and isthe cycle depth represented by the cycle depth signal from the cycledepth integrator 32, curve B is a reference wheel speed decelerating atthe rate of the reference deceleration in the deceleration switch 30,curve C is a reference wheel speed decelerating at the rate of thereference deceleration in the deceleration amplifier 34 and curve D isthe cycle depth reference supplied to the cycle depth comparator 40 fromthe deceleration amplifier 34, the acceleration switch 36 and the wheelspeed component from the conductor 28.

The wheel lock control system becomes operative to control the vehiclebraking when the brakes are applied and the wheel deceleration exceedsthe deceleration reference of the deceleration switch 30. During wheellock control operation, the wheel lock control circuit has three modesof operation to provide optimum control of the wheel brakes duringbraking for all vehicle load and road surface conditions. in all threemodes, the deceleration switch 30 is singularly effective to provide fora brake release when the wheel deceleration exceeds the referencedeceleration therein representing an incipient wheel lock condition.

In the first mode of operation, the duration of the brake release periodis controlled solely by the deceleration switch 30. This mode of controlis generally associated with heavy vehicle loads and/or high coefficientof friction road surfaces wherein the cycle depth during a brake releasetends to be shallow and short. This mode of operation is illustrated inFIG. 3a wherein at time t₀, the wheel deceleration exceeds the referencedeceleration and the deceleration switch 30 supplies the control signalto the release inhibit circuit 38 to terminate the release inhibitsignal and to the cycle depth comparator 40 which generates a brakerelease signal to effect energization of the release solenoid 44 torelease the vehicle brakes. Due to the delays in the brake system, thebrake pressure will continue to increase after the release solenoid 44is energized causing the wheel deceleration to increase beyond thereference deceleration of the deceleration switch 30. This is reflectedin the cycle depth signal (curve A). At time t₁, the cycle depthmagnitude becomes greater than the cycle depth reference (curve D). Attime t₂, the wheel deceleration stops in response to the decreased brakepressure and begins to accelerate toward vehicle speed. At time t₃, thecycle depth decreases to below the cycle depth reference (curve D).However, the brake release signal is continued at the output of thecycle depth comparator 40 in response to the control signal beinggenerated by the deceleration switch 30. At time t₄, the wheel speeddeviation from the reference speed (curve B) decelerating at thereference rate of the reference deceleration of the deceleration switch30 is reduced to zero and the deceleration switch 30 terminates thecontrol signal. At this time, the cycle depth reference signal suppliedto the cycle depth comparator 40 exceeds the cycle depth and thetermination of the control signal is effective to terminate the brakerelease signal at the output of the cycle depth comparator 40 to providefor brake reapplication. Termination of the brake release signal iseffective to reset the cycle depth signal to ground potential, theaverage wheel deceleration assumed to be less than the decelerationreference in the cycle depth integrator 32. Thereafter, the wheelacceleration stops and then begins to decelerate in response to theincreasing brake pressure. The cycle is repeated as previously describeduntil the vehicle comes to a stop, the vehicle brakes are released bythe vehicle operator, or other road surface or vehicle conditions areencountered which may shift the cyclic mode of the controller. This modeof operation, where the deceleration switch 30 is effective to controlthe duration of brake release, provides optimum wheel lock control forheavy vehicle load conditions and/or high coefficient of friction roadsurfaces.

In the second mode of operation, the duration of the brake release iscontrolled by the cycle depth signal output of the cycle depthintegrator 32. This mode of control is generally associated with mediumcoefficient of friction road surfaces and/or medium vehicle loads wherethe cycle depth during brake release tends to be deeper and longer thanin the first mode of operation. This mode of operation is illustrated inFIGS. 3b and 3c wherein at time t₀ in each figure, the wheeldeceleration exceeds the reference deceleration representing anincipient wheel lock condition and the deceleration switch 30 generatesthe control signal to terminate the release inhibit signal and initiatea brake release signal to initiate a brake release as in the first modeof operation. Upon release of the vehicle brake, the wheels continue todecelerate as illustrated by the cycly depth (curve A) as previouslydescribed as a result in the delay in the brake system. Due to the lowercoefficient of friction surface and lower vehicle loading, the cycledepth is generally greater and of longer duration than the cycle depthassociated with high coefficient of friction surfaces and high loadingconditions. At time t₁, the cycle depth (curve A) exceeds the magnitudeof the cycle depth reference (curve D). At time t₂, the wheeldeceleration stops and the whel begins to accelerate toward the vehiclespeed due to the decreased braking forces. In FIG. 3b, the wheel attainsa maximum acceleration less than the 2g reference in the accelerationswitch 36 and in FIG. 3c attains a maximum acceleration greater than the2g reference. At time t₃, the wheel speed deviation from the referencespeed (curve B) decelerating at the reference rate of the decelerationswitch 30 is reduced to zero and the deceleration switch 30 terminatesthe control signal. However, at the time t₃ the cycle depth is stillgreater than the cycle depth reference (curve D) determined by wheelspeed and the deceleration amplifier 34 in FIG. 3b and the accelerationswitch 36 in FIG. 3c so that the brake release signal from the cycledepth comparator 40 is maintained.

In the illustration of the second mode of operation provided by FIG. 3b,the cycle depth magnitude decreases to below the cycle depth referenceat time t₄. The brake release signal is therefore terminated by thecycle depth comparator 40 to effect brake reapplication. In theillustration of the second mode of operation provided by FIG. 3c, thewheel speed deviation from the reference speed (curve C) decelerating atthe reference deceleration of the deceleration amplifier 34 is reducedto zero at time t₄ and the output of the deceleration amplifier 34decreases to ground potential. However, the acceleration switch 30continues to supply its portion of the cycle depth reference through thegate 46 in response to the positive wheel acceleration exceeding the 2gacceleration reference therein. Therefore, the cycle depth referenceremains unaltered and the brake release signal from the cycle depthcomparator 40 is maintained. At time t₅ in FIG. 3c, the cycle depthdecreases to below the cycle depth reference at which time the brakerelease signal is terminated by the cycle depth comparator 40 to effectwheel brake reapplication.

Termination of the brake release signal in the FIGS. 3b and 3c iseffective to reset the cycle depth signal to ground potential, theaverage wheel deceleration assumed to be less than the decelerationreference in the cycle depth integrator 32. Thereafter, the wheelcontinues to accelerate for a time period and then again begins todecelerate in response to the applied brakes and the cycle is repeatedas previously described until the vehicle comes to a stop, the vehiclebrakes are released by the vehicle operator or other road surface orvehicle conditions are encountered which may shift the cyclic mode ofthe controller. The second mode of operation illustrated in FIGS. 3b and3c, where the cycle depth integrator is effective to control theduration of brake release, provides optimum wheel lock control formedium vehicle loading and/or medium coefficient of friction roadsurfaces.

Referring to FIG. 3d, there is illustrated the third mode of wheel lockcontol operation wherein the duration of the brake release is controlledby the release inhibit circuit 38. This mode of control is generallyassociated with low coefficient of friction surfaces and/or lightvehicle loading where the cycle depth during brake release tends to bedeeper and longer than in the first and second modes of operation. As inthe previous mode of operation, brake release is initiated at time t₀when the wheel deceleration exceeds the deceleration reference of thedeceleration switch 30 which generates the control signal to terminatethe release inhibit signal and initiate a brake release signal. At timet₁, the cycle depth (curve A) exceeds the magnitude of the cycle depthreference (curve D). At time t₂, the wheel deceleration stops and thewheel begins to accelerate due to decreased braking forces. Due to thelow coefficient surface and/or low vehicle loading, the positive wheelacceleration remains below the 2g reference of the acceleration switch36 but is greater than the positive acceleration reference of the relaseinhibit circuit 38. At time t₃, the wheel speed deviation from thereference speed (curve B) decelerating at the reference rate of thedeceleration switch 30 is reduced to zero and the deceleration switch 30terminates the control signal. However, the cycle depth is still greaterthan the cycle depth reference provided by the wheel speed signal andthe deceleration amplifier 34 so that the brake release signal ismaintained by the cycle depth comparator 40. At time t₄, the wheel speeddeviation from the reference speed (curve C) decelerating at thereference deceleration provided in the deceleration amplifier 34 isreduced to zero and the output of the deceleration amplifier 34 reducesto ground potential resulting in a decrease in the cycle depth referenceto a value determined by the wheel speed signal as the wheelacceleration is below the 2g acceleration reference of the accelerationswitch 36. The cycle depth continues to be greater than this lower valueof cycle depth reference so that the brake release signal is maintainedby the cycle depth comparator 40. At time t₅, the wheel accelerationdecreases to a level below the acceleration reference of the releaseinhibit circuit 38 representing the wheel speed approaching vehiclespeed. The release inhibit signal is therefore supplied to the cycledepth comparator 40 which terminates the brake release signal inresponse thereto to effect brake reapplication.

The third mode of operation illustrated in FIG. 3d provides optimumwheel lock control for light vehicle loading and/or low coefficient offriction road surfaces and cooperates with the first and second modes ofoperation to provide optimum wheel lock control operation for allvehicle load and road surface conditions.

During wheel lock control operation in the above described three modesof operation, the transistor 184 in the cycle depth integrator 32 isbiased nonconductive during the time period of the brake release signalfrom the cycle depth comparator 40 and therefore during the time periodof wheel brake release. During this time period, the capacitor 190,which was previously charged to the voltage across the resistor 188discharges through the resistor 188 at a rate related to thedeceleration reference provided when the transistor 184 is conducting.Upon termination of the brake release signal, the transistor 184 isagains biased conductive. The resulting current pulse supplied to thenegative input of the amplifier 192 as the capacitor 190 recharges issuch that the cycle depth signal is set to the value it would haveobtained if the transistor 184 were always conducting throughout theduration of the brake release signal and the deceleration referencesupplied therethrough were continually summed with the wheelacceleration signal.

If the average wheel deceleration is less than the referencedeceleration supplied while the transistor 184 is conducting, the cycledepth signal will be set to ground potential upon the termination ofeach one of the brake release signals. However, when the average wheeldeceleration exceeds the value of the reference deceleration suppliedwhen the transistor 184 is conducting and which represents the maximumpossible vehicle deceleration, the cycle depth signal is set to apositive level which is equal to the level it would have obtained if thereference deceleration were continually summed with the vehicleacceleration signal throughout the period of the brake release signal.If the average wheel deceleration continues to exceed the referencedeceleration level, the magnitude of the cycle depth signal continues tobe set to a progressively higher level until such time that during abrake release period, the output of the cycle depth integrator increasesto a value greater than the cycle depth reference supplied through theresistors 244 and 246 to maintain the brake release signal aftertermination of the control signal by deceleration switch 30 to allow forwheel speed recovery toward vehicle speed. This condition may exist, forexample, when the vehicle brakes are slowly applied by the vehicleoperator on low coefficient of friction surfaces so that the wheel cycledepth is similar as in braking on high coefficient surfaces and thesystem operates in the first mode of operation. When the cycle depthsignal attains a magnitude sufficient to maintain the release aftertermination of the control signal, the brake reapplication is generallycontrolled as in the third mode of control so that the wheel speed isagain allowed to increase to near vehicle speed. In this manner, thecycle depth integrator insures that the wheel will not approach a wheellock condition that would occur if the wheel deceleration were allowedto continue to exceed the maximum possible vehicle deceleration.

The remaining portion of the circuit of FIGS. 2a and 2b is directedtoward the self-check circuit for disabling the wheel lock controlsystem by commanding brake application and preventing brake release whencertain system faults are detected.

The self-check timer 50 receives one current input to the negative inputof the amplifier 288 from the output driver and shutdown circuit 42through a conductor 293, a diode 294 and a resistor 296. This current issupplied by the circuit 42 from the battery voltage B+, the resistor 280and the release solenoid 44 when the transistor 276 is biasednonconductive to provide for wheel brake application and if the releasesolenoid 44 is not faulted as by a short to ground or open circuit. Thiscurrent, which is insufficient to energize the release solenoid, isgreater than the bias current to the amplifier 288 through the resistor292 in the timer 50 so that the timer output is normally at groundpotential assuming no additional inputs to the positive input of theamplifier 292.

The self-check timer 50 also receives a current at the negative input ofthe amplifier 288 having a magnitude proportional to the cycle depthduring wheel lock control operation. This current is supplied by thecycle depth integrator 32 through a conductor 298, a resistor 300 andthe resistor 296.

Upon the initiation of a brake release wherein the transistor 276 isbiased conductive or upon the occurrence of an open or short to groundcircuit fault in the solenoid winding 44, the current to the timr 50through the conductor 293 is terminated and the output of the amplifier288 begins to integrate positive as a result of the current through theresistor 292 with the rate of increase being varied as a function of thecycle depth related current through the conductor 298. The output of thetimer integrates to the trigger level of the self-check comparator 54after a time representing an excessively long brake release period,which time is extended for increasing levels of the cycle depth. Uponthe termination of the release or the correction of the short or opencircuit release solenoid 44, the timr output integrates down to groundpotential.

The self-check timer also receives a sensor fault current at thepositive input of the amplifier 288 from a sensor continuity checkcircuit 48 which monitors the sensors 14 and 16 and supplies the sensorfault current upon the detection of an open circuit fault in either ofthe sensors 14 and 16.

The sensor continuity check circuit 48 includes an NPN transistor 302whose collector is coupled to the battery voltage B+ through a resistor304 and whose emitter is coupled to the positive input of the amplifier288 in the timer 50. The ungrounded side of each of the sensors 14 and16 are coupled to the base of the transistor 302 through respectiveresistors 308 and 306. A filter capacitor 310 is coupled between thebase of the transistor 302 and ground.

The base of the transistor 302 is normally at near ground potentialthrough the low impedance of the sensors 14 and 16. Consequently, whenthe sensors 14 and 16 are in an unfaulted condition, the transistor 302is biased nonconductive. If an open circuit should occur in one or bothof the sensors 14 or 16, the base bias of the transistor 302 isincreased by the voltage applied thereto from the regulated voltagesource Z+ through the resistors 76 and/or 94 and the resistors 306and/or 308. This voltage is sufficient to bias the transistor 302conductive to supply the sensor fault current to the positive input ofthe amplifier 288 in the self-check timer 50. This sensor fault current,when summed with the bias current through the resistor 292, alwaysexceeds the current to the negative input of the amplifier 288 so thatthe output of the amplifier integrates positive and attains the triggerlevel of the self-check comparator 54 after a specified time period.Correction of the sensor fault and termination of the sensor faultcurrent results in the output of the timer 50 integrating down to groundpotential.

The power supply fault current supplied by the regulated power supply 52when the battery voltage B+ decreases below the regulated voltage Z+, aspreviously described, is coupled to the positive input of the amplifier288 through the conductor 74. This power supply fault current functionsin the same manner as the sensor fault current to cause the output ofthe amplifier 288 to integrate positive to the trigger level of theself-check comparator 54 after a specified time period. When the B+voltage again increases to above the Z+ voltage, the power supply faultcurrent is terminated and the output of the timer 40 integrates down toground potential.

The output of the self-check timer 50 is coupled to the negative inputof an operational amplifier 312 in the self-check comparator 54 througha resistor 314. A bias current is also supplied to the negative inputthrough a resistor 316 coupled between Z+ and the negative inputterminal. A first feedback resistor 318 is coupled between the output ofthe amplifier 312 and its positive input and a second feedback resistor320 is coupled between its output and its positive input through a diode322, the anode of the diode being coupled to the output. A current issupplied to the positive input of the amplifier 312 from the outputdriver and shutdown circuit 42 through a conductor 324, a resistor 326coupled to the cathode of the diode 322, and the resistor 320 when thewheel lock control system generates a brake release signal. The voltageat the cathode of the diode 322 is limited to the regulated voltage Z+by a diode 327 coupled between the diode 322 and the regulated voltageZ+. The current is supplied by the output driver and shutdown circuit 42from the battery voltage B+ and a resistor 328 coupled between thebattern voltage B+ and the conductor 324. The conductor 324 is alsocoupled to the collector of the transistor 266 so that upon thegeneration of a brake release signal and the resulting conduction of thetransistor 266, the current supplied to the comparator 54 through theconductor 324 is terminated. The current to the comparator through theconductor 324 is therefore supplied only when the wheel lock controlsystem logic output is a brake supply signal (absence of a brake releasesignal).

The circuit values are selected such that in the absence of an outputfrom the timer 50, the current supplied to the positive input of theamplifier 312 through the conductor 324 and the resistor 320 when thewheel lock control system output is a brake apply logic is greater thanthe current to its negative input through the resistor 316, and the sumof the currents to its positive input through the resistor 318 and thediode 322 and the resistor 320 when its output is at a high levelexceeds the current to its negative input through the resistor 316.Therefoe, when the wheel lock control circuit is first energized andprovides the brake apply logic, the output of the amplifier 312 shiftsto a high voltage which functions to latch the output at the highvoltage by the feedback through the resistors 318 and 320. This normallyhigh output is coupled to the transistor 268 through the conductor 282to bias the transistor 268 conducting so as to enable the output driverand shutdown circuit to effect brake release during the period of abrake release signal provided by the cycle depth comparator 40. Theamplifier 312 in the self-check comparator circuit 54 is shifted fromits normally high state to ground potential only upon the receipt of acurrent from the selfcheck timer 50 having a magnitude greater than thetrigger level of the comparator 54 which is the difference between thefeedback currents to the positive input of the amplifier 312 and thebias current to the negative input through the resistor 316. Theself-check timer 50 integrates to provide a signal of this magnitudeafter a specified time duration after the detection of a fault or abrake release. When the output of the self-check timer 50 attains thetrigger lever of the comparator 54, the output of the amplifier 312shifts to ground potential which comprises the shutdown signal which iseffective to bias the transistor 268 in the output driver and shutdowncircuit 34 nonconductive to deenergize the brake release solenoid 44 toprevent wheel brake release by the wheel lock control system. Theshutdown signal is provided by the comparator 54 as long as the outputof the self-check timer 50 remains above the trigger level of thecomparator 54.

The response of the self-check comparator 54 to the termination of thefault and subsequent decrease in the output of the timer 50 to below thetrigger level of the comparator is dependent upon whether or not thewheel lock control circuit is generating a brake release signal. If thewheel lock control circuit is in a brake apply mode, termination of thefault and subsequent reduction of the output of the self-check timer 50to below the trigger lever of the comparator 54 results in theself-check comparator 54 terminating the shutdown signal by shifting itsoutput again to the positive voltage level as a result of the currentsupplied to the positive input of the amplifier 312 from the outputdriver and shutdown circuit 42 through the conductor 324 during thebrake apply mode. However, if the fault is terminated during the timeperiod that the wheel lock control system is generating a brake releasesignal and therefore in a brake release mode, there are no currentssupplied to the positive input of the amplifier 312 and the bias currentthrough the resistor 316 maintains the output of the amplifier at groundpotential. Therefore, the self-check comparator is maintained latchedinto its low state to continue the shutdown signal. Therefore, theoutput driver and shutdown circuit 42 will be ineffective to provide forbrake release for the duration of the brake release signal from thecycle depth comparator 40. Upon termination of the brake release logicsignal by the wheel lock control circuit, the shutdown signal isterminated by the resulting current supplied to the amplifier 312through the conductor 324. In this manner, the self-check comparator 54provides for non-latching shutdown of the wheel lock control system forthe duration of the fault if the fault is corrected during a systembrake apply mode and provides for latching shutdown of the wheel lockcontrol system for the longer one of the fault condition or the brakerelease signal if the fault condition exists at least momentarily withthe brake release signal.

The dynamic sensor check circuit 58 includes a pair of PNP transistors330 and 332. The speed signal from the tachometer 22 is coupled to thebase of the transistor 330 and to the emitters of each of thetransistors 330 and 332 through a diode 334 and a resistor 336. Thespeed signal from the tachometer 24 is coupled to the base of thetransistor 332 and to the emitters of the transistors 330 and 332through a diode 338 and the resistor 336. The emitters are also groundedthrough a resistor 340.

The diodes 334 and 338 apply the maximum wheel speed signal across thevoltage divider formed by the resistors 336 and 340. When the highestwheel speed exceeds the lowest wheel speed by a predetermined amount,such as 17 mph, the transistor 330 or 332 having the lowest wheel speedapplied to its base is biased conductive to supply a current to thepositive input of the cycle depth comparator 40 through a conductor 342to effect the generation of a brake release signal and the energizationof the brake release solenoid 44. After the time period of theself-check timer 50, the self-check comparator 54 provides the shutdownsignal to disable the wheel lock control circuit. In this manner, thesystem is disabled in response to a sensor 14 or 16 that is inoperativefor reasons other than loss of continuity.

A warning is provided upon the generation of a shutdown signal toindicate a fault condition to the vehicle operator. The fault signal iscoupled to the base of an NPN transistor 344 in the lamp driver 56through a resistor 346. The collector of the transistor is coupled tothe base of a Darlington transistor 348 whose emitter is grounded. Thecollectors of the transistors 344 and 348 are coupled to a lamp 350through respective resistors 352 and 354, the remaining side of the lampbeing coupled to the battery voltage B+. Upon generation of the shutdownsignal, the transistor 344, which is normally conducting, is biased offto bias the transistor 348 conductive to energize the lamp 350 andprovide the indication.

The description of the preferred embodiment of this invention for thepurpose of illustrating the principles thereof is not to be consideredas limiting or restricting the invention since many modifications may bemade by the exercise of skill in the art without departing from thescope of the invention.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A wheel lock controlsystem for a vehicle with braked wheels comprising:means effective toprovide a speed signal representing wheel speed; a deceleration switchresponsive to the speed signal effective to provide a control signalwhich is initiated when the rate of change of wheel speed exceeds apredetermined threshold deceleration indicative of incipient wheel lock;a cycle depth integrator effective to generate a cycle depth signal; andcontrol means responsive to the control signal and the cycle depthsignal for effecting brake release during the period of the controlsignal and during the time period the cycle depth signal exceeds areference value, the cycle depth integrator including; means responsiveto the speed signal effective to provide an acceleration signalrepresenting wheel acceleration, means effective during brakeapplication to provide a deceleration reference signal representing amaximum vehicle deceleration, the deceleration reference signal beingterminated during brake release, integrating means effective tointegrate the sum of the acceleration signal and the decelerationreference signal to provide the cycle depth signal, the cycle depthsignal being a composite signal comprises of a first porion representingthe amount that the time integral of the deceleration reference signalexceeds the wheel speed during brake application and a second portionwhich is the integral of wheel acceleration during brake release, andmeans responsive to brake reapplication effective to set the cycle depthsignal output of the integrating means to a value equal to the value thecycle depth signal would have obtained if the deceleration referencesignal were continually summed with the acceleration signal, the cycledepth signal having a magnitude that increases to a value greater thanthe reference value after repeated brake release and application cycleswhen the average wheel deceleration exceeds the maximum possible vehicledeceleration representing the wheel approaching a locked condition tomaintain the wheel brake released to allow the wheel to acceleratetoward vehicle speed to thereby prevent wheel lock.
 2. A wheel lockcontrol system for a vehicle with braked wheels comprising:meanseffective to provide a speed signal representing wheel speed; adeceleration switch responsive to the speed signal effective to providea control signal wich is initiated when the rate of change of wheelspeed exceeds a predetermined threshold deceleration indicative ofincipient wheel lock; a cycle depth integrator effective to generate acycle depth signal; and control means responsive to the control signaland the cycle depth signal for effecting brake release during the periodof the control signal and during the time period the cycle depth signalexceeds a reference value, the cycle depth integrator including; meansresponsive to the speed signal effective to provide an accelerationsignal representing wheel acceleration, means effective to provide adeceleration reference signal representing a maximum vehicledeceleration, a summing junction, means effective to couple theacceleration signal and the deceleration reference signal to the summingjunction, the summing junction providing a output signal that is thealgebraic sum of the acceleration signal and the deceleration referencesignal, integrating means coupled to the summing junction effective tointegrate the sum of te acceleration signal and the decelerationreference signal to provide the cycle depth signal, the means effectiveto provide a deceleration reference signal including a voltage source, aseries circuit comprised of a resistor and a switch means coupledbetween the voltage source and the summing junction, and a capacitorcoupled in parallel with the resistor, the switch means being coupled tothe contrl means and being biased nonconductive thereby during theperiod of a brake release and biased conductive thereby for the periodof brake application, the cycle depth signal being a composite signalincluding a first portion representing the amount that the time integralof the deceleration reference signal exceeds the wheel speed duringbrake application and a second portion which is the integral of wheelacceleration during brake release, the capacitor being charged duringperiods of brake application and discharging through the resistor duringperiods of brake release at a rate related to the deceleration referencesignal so that a charge pulse is supplied to the summing junction whenthe switch means is biased conductive to set the cycle depth signaloutput of the integrating means to a value equal to the value of thecycle depth signal would have obtained if the switch means werecontinually biased conductive to supply the deceleration referencesignal, the cycle depth signal having a magnitude that increases to avalue greater than the reference value after repeated brake release andapplication cycles when the average wheel deceleration exceeds themaximum possible vehicle deceleration representing the wheel approachinga locked condition to maintain the wheel brake released to allow thewheel to accelerate to vehicle speed to thereby prevent wheel lock.
 3. Awheel lock control system for a vehicle with braked wheelscomprising:means effective to provide a speed signal representing wheelspeed; a deceleration switch responsive to the speed signal effective toprovide a control signal wich is initiated when the rate of change ofwheel speed exceeds a predetermined threshold deceleration indicative ofincipient wheel lock and terminated when the wheel speed again equals afirst reference wheel speed decelerating at the rate of thepredetermined threshold deceleration; a cycle depth integratorresponsive to the speed signal effective to generate a cycle depthsignal; a deceleration amplifier for providing a first reference signalwhen the wheel deceleration exceeds a reference deceleration level lessthan the predetermined threshold deceleration and terminating the firstreference signal when the wheel speed again equals a second referencewheel speed having a deceleration equal to the reference decelerationlevel; an acceleration switch responsive to wheel speed for providing asecond reference signal during the time period the rate of change ofwheel speed exceeds a specified positive acceleration level; meansresponsive to wheel speed effective to generate a third reference signalhaving a magnitude related to the wheel speed; means effective to sumone of the first or second reference signals having the greatestmagnitude with the third reference signal to provide a cycle depthreference signal; a release inhibit switch effective to generate arelease inhibit signal during both the absence of a control signal andwhen the wheel acceleration is below a predetermined low levelrepresenting the wheel speed approaching vehicle speed; comparator meanseffective to generate a brake release signal when the sum of the cycledepth signal and the control signal is greater than the sum of the cycledepth reference signal and the release inhibit signal, the releaseinhibit signal being greater than the maximum value of the cycle depthsignal and the control signal being greater than the maximum value ofthe cycle depth reference signal so that the deceleration switch issingularly effective to initiate a brake release signal, the cycle depthsignal having a magnitude representing the wheel speed change during thebrake release signal; and control means responsive to the brake releasesignal for effecting wheel brake release during the period of the brakerelease signal, the deceleration switch being effective to control theduration of wheel brake release during a first mode of operation of thewheel lock control system, the cycle depth integrator being effective tocontrol the duration of a wheel brake release during the second mode ofoperation of the wheel lock control system and the release inhibitswitch being operative to control the duration of the wheel brakerelease in a third mode of operation of the wheel lock control system.4. A wheel lock control system for a vehicle with braked wheelscomprising:means effective to provide a speed signal representing wheelspeed; a deceleration switch responsive to the speed signal effective toprovide a control signal whichis initiated when the rate of change ofwheel speed exceeds a predetermined threshold deceleration indicative ofincipient wheel lock; a cycle depth integrator effective to generate acycle depth signal; and control means responsive to the control signaland the cycle depth signal for effecting brake release during the periodof the control signal and during the time period the cycle depth signalexceeds a certain value, the certain value having a first level greaterthan all values of the cycle depth signal during brake application andhaving a second lower level during brake release so that thedeceleration switch is singularly effective to initiate brake release,the cycle depth integrator including; means responsive to the speedsignal effective to provide an acceleration signal representing wheelacceleration or deceleration, means effective during brake applicationto provide a deceleration reference signal representing a maximumvehicle deceleration, the deceleration reference signal being terminatedduring brake release, integrating means effective to integrate the sumof the acceleration signal and the deceleration reference signal toprovide the cycle depth signal, the cycle depth signal being a compositesignal comprised of a first portion representing the amount that thetime integral of the deceleration reference signal exceeds the wheelspeed during brake application and a second portion which is theintegral of wheel acceleration during brake release, the cycle depthsignal having a value below the second level of the reference value upontermination of a control signal generated by the deceleration switch ina first range of road coefficient and wheel load conditions so that thedeceleration switch is solely effective to provide optimum wheel lockcontrol in the first range of conditions and the cycle depth signalhaving a value greater than the second level of the reference value upontermination of a control signal generated by the deceleration switch ina second range of road coefficient and wheel load conditions so that thecycle depth integrator is effective to extend the duration of the brakerelease initiated by the deceleration switch to provide for optimumwheel lock control in the second range of conditions, and meanseffective upon brake reapplication to set the integrating means so thatthe cycle depth signal provided thereby has a value equal to the valuethe cycle depth signal would have obtained if the deceleration referencesignal were continually summed with the acceleration signal, the cycledepth signal having a magnitude that increases to a value greater thanthe second level of the reference value over a series of brake releaseand application cycles when the average wheel deceleration exceeds themaximum possible vehicle deceleration representing the wheel approachinga locking condition to extend the brake release upon termination of thecontrol signal from the deceleration switch to allow the wheel toaccelerate toward vehicle speed to thereby prevent wheel lock.